groupFoldBy<K, G> method

Map<K, G> groupFoldBy<K, G>(
  1. K keyOf(
    1. T element
  2. G combine(
    1. G? previous,
    2. T element

Groups elements by keyOf then folds the elements in each group.

A key is found for each element using keyOf. Then the elements with the same key are all folded using combine. The first call to combine for a particular key receives null as the previous value, the remaining ones receive the result of the previous call.

Can be used to group elements into arbitrary collections. For example groupSetsBy could be written as:

    (Set<T>? previous, T element) => (previous ?? <T>{})..add(element));


Map<K, G> groupFoldBy<K, G>(
    K Function(T element) keyOf, G Function(G? previous, T element) combine) {
  var result = <K, G>{};
  for (var element in this) {
    var key = keyOf(element);
    result[key] = combine(result[key], element);
  return result;